Random telegraph signal noise reduction scheme for semiconductor memories

ABSTRACT

Embodiments are provided that include a method including providing a first pulsed gate signal to a selected memory cell, wherein the pulsed gate signal alternates between a first voltage level and a second voltage level during a time period and sensing a data line response to determine data stored on the selected memory of cells. Further embodiments provide a system including a memory device, having a regulator circuit coupled to a plurality of access lines of a NAND memory cell, and a switching circuit configured to sequentially bias at least one of the plurality of the access lines between a first voltage level and a second voltage level based on an input signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/020,460, which was filed on Jan. 25, 2008, and issued on Mar. 29,2011 as U.S. Pat. No. 7,916,544.

BACKGROUND

1. Field of the Invention

Embodiments of the invention relate generally to the field of memorydevices and more particularly, to reducing the effect of randomtelegraph signal noise (RTS noise) in semiconductor memories.

2. Description of the Related Art

Flash memory is a non-volatile memory that can be electrically erasedand reprogrammed. It is primarily used in memory cards, USB flashdrives, and the like for storage of data in computer systems. Generally,flash memory stores information in an array of floating gatetransistors, called “cells”, each of which traditionally stores one bitof information that is represented as a “0” or a “1”. Each cell ischaracterized by a threshold voltage (Vt) that varies based on the datastored in the cell. For example, during program and erase operations,charge is added or removed from a floating gate to change the cell'sthreshold voltage, thereby defining whether the cell is programmed orerased. During a read operation, a read voltage is applied to the celland a response of the cell (e.g., a current across the cell) ismonitored to determine whether the threshold voltage is above or belowthe read voltage. In other words, the read operation can determine ifthe cell is programmed as a 1 or a 0 value. Multi-level cells mayinclude multiple threshold voltage ranges that are representative ofadditional values, such as two or more bits of information.

Flash memories may also employ a verify operation that ensures that eachcell is programmed as a given state, such as a 1 or a 0. The verifyoperation may provide a sufficient margin between the 1 and 0 statessuch that a cell is charged to a given range and does not charge to anintermediate state where the cell may be read incorrectly. However, whenthe memory cells are programmed at one temperature and are read out atanother temperature, the margin may decrease, potentially causing thevalue of the cell to be read incorrectly. For example, when the readoperation is executed during a first period at a first temperature, thedata may tend to be a 0, and when the read operation is executed duringa second period at a second temperature, the data may tend to be a 1.This is prevalent where a word line voltage is a constant voltage valueover the range of temperatures. The variations may be attributed torandom telegraph noise (RTS noise) and the resulting time dependency ofthe current that passes through the memory cell. The RTS noise can beattributed to the trap and detrap of electrons or the recombination ofelectrons with holes. Unfortunately, the presence of the RTS noise andthe resulting inaccuracies in reading the memory cells may produceinaccurate and/or less reliable memory devices.

Embodiments of the present invention may be directed to one or more ofthe problems set forth above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that illustrates a processor-based devicehaving a memory that includes memory devices fabricated in accordancewith one or more embodiments of the present invention;

FIG. 2 is a block diagram that illustrates a memory device having amemory array fabricated in accordance with one or more embodiments ofthe present invention;

FIG. 3 is a schematic diagram of a NAND flash memory array having memorycells fabricated in accordance with one or more embodiments of thepresent invention;

FIG. 4 is a graph that illustrates the variation in a cell current overvarious periods;

FIG. 5 is a schematic diagram of a column of the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 6 is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 7A is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 7B is a schematic diagram of a regulator circuit in accordance withone or more embodiments of the present invention.

FIG. 7C is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 7D is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 7E is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 8 is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 9A is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 9B is a schematic diagram of a regulator circuit in accordance withone or more embodiments of the present invention;

FIG. 9C is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 9D is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention;

FIG. 10 is a timing diagram of operating the NAND flash memory inaccordance with one or more embodiments of the present invention; and

FIGS. 11A-11E are schematic diagrams of devices that can employtechniques in accordance with one or more embodiments of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

As discussed in further detail below, the disclosed systems and methodsrelate to a random telegraph noise (RTS noise) reduction scheme forsemiconductor memory devices. In certain embodiments, signals to theword line and/or the select gate are pulsed to enable trapped electronsin the semiconductor to recombine with holes (i.e., detrap) during anaccumulation period, to reduce the electron trap for the next inversionperiod. Detrap should reduce the RTS noise, thereby increasing theverify margin between states. In other words, the uncertainty that ispresent due to RTS noise during read and verify operations should bereduced. Before a detailed discussion of the system and methodsdescribed in accordance with various embodiments of the presentinvention, it may be beneficial to discuss embodiments of memory devicesthat may incorporate the devices described herein, in accordance withembodiments of the present technique.

Turning now to the figures, FIG. 1 includes a block diagram depicting aprocessor-based system, generally designated by reference numeral 10.The system 10 may be any of a variety of types such as a computer,pager, cellular phone, personal organizer, control circuit, etc. In atypical processor-based device, a processor 12, such as amicroprocessor, controls the processing of system functions and requestsin the system 10. Further, the processor 12 may comprise a plurality ofprocessors that share system control.

The system 10 typically includes a power supply 14. For instance, if thesystem 10 is a portable system, the power supply 14 may advantageouslyinclude permanent batteries, replaceable batteries, and/or rechargeablebatteries. The power supply 14 may also include an AC adapter, so thatthe system 10 may be plugged into a wall outlet, for instance. The powersupply 14 may also include a DC adapter such that the system 10 may beplugged into a vehicle cigarette lighter, for instance.

Various other devices may be coupled to the processor 12 depending onthe functions that the system 10 performs. For instance, a userinterface 16 may be coupled to the processor 12. The user interface 16may include buttons, switches, a keyboard, a light pen, a mouse, and/ora voice recognition system, for instance. A display 18 may also becoupled to the processor 12. The display 18 may include an LCD display,a CRT, LEDs, and/or an audio display, for example.

Furthermore, an RF sub-system/baseband processor 20 may also be coupleto the processor 12. The RF sub-system/baseband processor 20 may includean antenna that is coupled to an RF receiver and to an RF transmitter(not shown). A communications port 22 may also be coupled to theprocessor 12. The communications port 22 may be adapted to be coupled toone or more peripheral devices 24 such as a modem, a printer, acomputer, or to a network, such as a local area network, remote areanetwork, intranet, or the Internet, for instance.

Because the processor 12 controls the functioning of the system 10 byimplementing software programs, memory is used in conjunction with theprocessor 12. Generally, the memory is coupled to the processor 12 tostore and facilitate execution of various programs. For instance, theprocessor 12 may be coupled to system memory 26, which may includevolatile memory, such as Dynamic Random Access Memory (DRAM) and/orStatic Random Access Memory (SRAM). The system memory 26 may alsoinclude non-volatile memory, such as read-only memory (ROM), EEPROM,and/or flash memory to be used in conjunction with the volatile memory.As discussed in further detail below, the system memory 26 may includeone or more memory devices, such as flash memory devices, that include afloating gate memory array fabricated and implementing techniques inaccordance with one or more embodiments of the present invention.

FIG. 2 is a block diagram that illustrates a flash memory device 30 thatmay be included as a portion of the system memory 26 of FIG. 1. As willbe discussed in further detail with respect to FIG. 3, the flash memorydevice 30 may be a NAND flash memory device. The flash memory device 30generally includes a memory array 32. The memory array 32 generallyincludes many rows and columns of conductive traces arranged in a gridpattern. “Access lines” are used to access cells and generallycorrespond to the rows or “row lines” of the memory array 32. In theconventional art, they are generally referred to as “word lines.” “Datalines” generally correspond to the columns or “column lines.” In theconventional art, they are generally referred to as “digit (e.g., bit)lines.” The size of the memory array 32 (i.e., the number of memorycells) will vary depending on the size of the flash memory device 30.

To access the memory array 32, a row decoder block 34 and a columndecoder block 36 are provided and are configured to receive andtranslate address information from the processor 12 via the address bus38 and the address buffer 40 and to access a particular memory cell inthe memory array 32. A sense amplifier block 42, having a plurality ofthe sense amplifiers, is also provided inline with the column decoder 36and the memory array 32. The sense amplifier block 42 senses andamplifies individual values stored in the memory cells. A row driverblock 46 is provided to activate a selected word line in the memoryarray according to a given row address.

An internal voltage source 44, such as a voltage generator, is providedto deliver voltages for use within the memory device 30. The internalvoltage source 44 may provide voltage levels for program, program, read,verify, and erase operations. The internal voltage source 44 may includea trimming circuit to accurately regulate the voltage level output bythe internal voltage source 44.

During read and program operations, data may be transferred to and fromthe flash memory device 30 via the data bus 48. The coordination of thedata and address information may be conducted through a control circuit50. The control circuit 50 may be configured to receive control signalsfrom the processor 12 via the control bus 52. A command buffer 54 may beconfigured to temporarily store commands of the control circuit 50. Thecontrol circuit 50 is coupled to each of the row decoder block 34, thecolumn decoder block 36, the address buffer 40, the sense amplifierblock 42, the internal voltage generator 44, the row driver block 46,and the command buffer 54, and is generally configured to coordinatetiming and control among the various circuits in the flash memory device30.

FIG. 3 illustrates an embodiment of the memory array 32, of FIG. 2. Inthe illustrated embodiment, the memory array 32 includes a NAND memoryarray 56. The NAND memory array 56 includes word lines WL(0)-WL(M) andintersecting local bit lines BL(0)-BL(N). As will be appreciated, forease of addressing in the digital environment, the number of word linesWL and the number of bit lines BL are each a power of two (e.g., 256word lines (WL) by 4,096 bit lines (BL)). The local bit lines BL arecoupled to global bit lines (not shown) in a many-to-one relationship.

The NAND memory array 56 includes a floating gate transistor 58 locatedat each intersection of a word line (WL) and a local bit line (BL). Thefloating gate transistors 58 serve as non-volatile memory cells forstorage of data in the NAND memory array 56, as previously discussed. Aswill be appreciated, each floating gate transistor includes a source, adrain, a floating gate, and a control gate. The control gate of eachfloating gate transistor 58 is coupled to a respective word line (WL).The floating gate transistors 58 are connected in series, source todrain, to form a NAND string 60 formed between gate select lines.Specifically, the NAND strings 60 are formed between the drain selectline (SGD) and the source select line (SGS). The drain select line (SGD)is coupled to each NAND string 60 through a respective drain select gate62. Similarly, the source select line (SGS) is coupled to each NANDstring 60 through a respective source select gate 64. The drain selectgates 62 and the source select gates 64 may each comprise a field-effecttransistor (FET), for instance. A column of the memory array 56 includesa NAND string 60 and the source select gate 64 and drain select gate 62connected thereto. A row of the floating gate transistors 58 are thosetransistors commonly coupled to a given word line (WL).

The source of each source select gate 64 is connected to a common sourceline (SL). The drain of each source select gate 64 is coupled to thesource of a floating gate transistor 58 in a respective NAND string 60.The gate of each source select gate 64 is coupled to the source selectline (SGS).

The drain of each drain select gate 62 is connected to a respectivelocal bit line (BL) for the corresponding NAND string 60. The source ofeach drain select gate 62 is connected to the drain of a floating gatetransistor 58 of a respective NAND string 60. Accordingly, asillustrated in FIG. 3, each NAND sting 60 is coupled between arespective drain select gate 62 and source select gate 64. The gate ofeach drain select gate 62 is coupled to the drain select line (SGD).

During operation of the flash memory device 30, multiple voltages aregenerated within the memory device 30 to accomplish various tasks. Forexample, the memory device 30 may employ multiple voltage levels appliedto the word lines, bit lines, and the like, to program, read, erase andverify values stored in the cells of the memory array 32. Specifically,the flash memory device 30 may employ a verify operation that ensuresthat each cell is programmed as in a given state, such as a 1 or a 0.The verify operation may provide a sufficient margin between the 1 and 0states such that a selected cell is charged into a given range and doesnot charge to an intermediate state that may cause the selected cell tobe read incorrectly. However, when the memory cells are programmed atone temperature and are read out at another temperature, the margin maydecrease, leading to a possibility that the value of the selected cellmay be read incorrectly.

For example, as illustrated in FIG. 4, when the read operation isexecuted during a first period 66 at a first temperature, the level of asensed current (Icell) across the selected cell may be indicative of a 0value, and when the read operation is executed during a second period 68at a second temperature, the level of the sensed current (Icell) acrossthe selected cell may be indicative of a 1 value. This is prevalentwhere a word line voltage is a constant voltage value over the range oftemperatures. The variations may be attributed, at least partially, torandom telegraph noise (RTS noise) and the resulting time dependency ofthe current that passes through the memory cell. The RTS noise can beattributed to the trap and detrap of electrons or the recombination ofelectrons with holes. Unfortunately, the presence of the RTS noise andthe resulting inaccuracies in reading the memory cells may produceinaccurate results and/or reduce the reliability of the memory device.The following techniques for reducing RTS noise are discussed in thecontext of a NAND flash memory device and, to simplify the discussion,are discussed in the context of a single column of a NAND memory array.

FIG. 5 is a schematic diagram of a column 70 of a NAND memory array. Thecolumn 70 includes a floating gate transistor (i.e., a cell) 72 locatedat each intersection of a word line (WL) (e.g., WL0, WL1, WL2, WL3) anda local bit line (BL). The control gate of each cell 72 is coupled to arespective word line (WL).

The cells 72 are connected in series, source to drain, to form a NANDstring 74 formed between a drain select line (SGD) and a source selectline (SGS). The drain select line (SGD) is coupled to the NAND string 74through a respective drain select gate transistor 76, and the sourceselect line (SGS) is coupled to the NAND string 74 through a respectivesource select gate transistor 78. The source of the source select gatetransistor 78 is connected to a common source line (SL), the drain ofthe source select gate transistor 78 is coupled to the source of a cell72 of the NAND string 74, and the gate of the source select gatetransistor 78 is coupled to the source select line (SGS). The drain ofthe drain select gate transistor 76 is connected to the local bit line(BL) of the NAND string 74, the source of the drain select gatetransistor (SGD) 76 is connected to the drain of a cell 72 of the NANDstring 74, and the gate of the drain select gate transistor 76 iscoupled to the drain select line (SGD).

The local bit line (BL) is coupled to a sense amplifier 80. The senseamplifier 80 includes a PASS transistor 82, a PREB transistor 84, and aninverter 86. The bit line (BL) is coupled to a source of the PASStransistor 82. The gate of the PASS transistor 82 is coupled to a PASSline (PASS), and the drain of the PASS transistor 82 is coupled to adata-in node (DIN). A drain of a PREB transistor 84 is coupled to thedata-in node (DIN), a source of the PREB transistor 84 is coupled to acommon voltage (Vcc), and a floating gate of the PREB transistor 84 iscoupled to a PREB line (PREB). The data-in node (DIN) is coupled to aninput of the inverter 86 that is coupled to an output enable (OE)signal. The inverter 86 has an output on a data-out line (DOUT).

FIG. 6 is a timing diagram that illustrates signals in accordance withan operation of sensing (e.g., reading or verifying) cell data. Thisfigure may be reviewed in conjunction with FIG. 5, for example. The bitline (BL) is precharged to about 1V(volt) with the PASS line (PASS)biased with 2V. At the beginning of a discharging period (Tbl), the wordlines (WL) and the select gate signals (SGS and SGD) are transition froma low voltage to a high voltage. For example, the word line of theselected cell (WL1) is raised to about 1V. The word line to theunselected cells (WL0, WL2, and WL3) and the select gate signals (SGSand SGD) are raised to a high level, such as 5V.

After the bit line (BL) is precharged, the PASS line (PASS) is groundedto disconnect the data-in node (DIN) from the bit line (BL). Dependingon the memory data, the bit line (BL) voltage is reduced or remainshigh. For example, where the data stored on the selected cell is “1” thebit line (BL) voltage lowers, as indicated by the dashed line thatlowers to a “1” state. Where the data stored on the selected cell is “0”the bit line (BL) voltage remains high, at 1V, as indicated by the bitline (BL) signal maintaining a “0” state. During this period, thedata-in node (DIN) voltage is forced to Vcc, for example 3V. At the endof the discharging period (Tbl), the PASS line (PASS) voltage is raisedto 1.5V.

Where the data stored on the cell is “0”, if the bit line (BL) ismaintained at 1V, the gate-source voltage (Vgs) of the PASS transistor82 is 0.5V (0.5V=1.5V-1V) and the PASS transistor 82 is in an off state,assuming the threshold voltage (Vt) of the PASS transistor 82 is about1V. The data-in node (DIN) voltage is also maintained high as 3V. Thedata-out line (DOUT) voltage goes low with the output enable (OE) high.The data on the data-out line (DOUT) is transferred to a DQ pad and theresultant data represents “0”.

Where the data stored on the cell is “1”, if the bit line (BL) isdischarged according to the cell current (Icell), the bit line (BL)voltage lowers to about 0V. The gate-source voltage (Vgs) of the PASStransistor 82 is 1.5V (1.5V=1.5V-0V) so that the PASS transistor 82 ison. The data-in node (DIN) voltage is discharged to a bit-line (BL)capacitance, resulting in a data-in (DIN) voltage of 0V. The data-outline (DOUT) transitions to high with the output enable (OE) high. Thedata on the data-out line (DOUT) is transferred to the DQ pad and theresultant data represents “1”.

It should be noted that a similar technique may be used for a verifyoperation, wherein the voltage on the word line of the selected cell(WL1) is driven to a higher voltage, such as 1.5V.

As discussed previously, if the memory device 30 operates in accordancewith the embodiments of FIG. 6, it may be susceptible to RTS noise thatis attributable, at least in part, to the trap and detrap of electronsor a recombination of electrons with holes in the cell. The followingembodiments include a method and system that includes providing a pulsedsignal (rather than a constant signal) to at least one of the pluralityof cells (e.g., the selected cell) and sensing a bit line current todetermine whether data is stored on one of the plurality of cells. Thepulsed signal(s) alternates between a high voltage level and a lowvoltage level to reduce the RTS noise.

FIG. 7A is a timing diagram that illustrates signals in accordance withan embodiment of a method of operating the memory device 30. The methodincludes providing a pulsed (e.g., sequentially biased) signal to theselected cell. For example, in the illustrated embodiment, the word lineto the selected cell (WL1) is pulsed between a low state and a highstate during the discharging period (Tbl). The discharging period (Tbl)is extended for a period such that the word line signal to the selectedcell (WL1) is in the high state for a total time that is approximatelyequal to the time period that the word line to the selected cell (WL1)is held high in an embodiment where they are not being pulsed. In otherwords, the total time the word line to the selected cell (WL1) is highin the discharge period (Tbl) is about the same amount of time the wordline to the selected cell (WL1) is high in the illustration of FIG. 6.Where the word line to the selected cell (WL1) is pulsed, the currentacross the bit line (BL) is integrated over the discharging period (Tbl)to determine whether data is stored on the cell and/or to identify datastored on the cell.

During the discharging period (Tbl), the pulsed word line signal to theselected cell (WL1) alternates between a high voltage level, where thecell is on, and a low voltage level, where the cell is off. For example,in the illustrated embodiment, the high voltage level is about 1V andthe low voltage level is about 0V. While the word line signal to theselected cell (WL1) is supplied with the low voltage level, the memorycell connected to (WL1) gets accumulated to detrap electrons that trapwhile the word line signal to the selected cell (WL1) is supplied withthe high voltage level. The other signals operate similar to thosediscussed above with regard to FIG. 6, and incorporate a longerdischarging period (Tbl). For example, the word lines to the unselectedcells (WL0, WL2, and WL3) and the select gate signals (SGS and SGD) areraised to a high level, such as 5V, and are not pulsed (i.e., they areheld at a constant voltage level).

Further, it is noted that the word line (WL1) is pulsed during a firsttime period (T(on)) and maintained in a low state during a second period(T(off)) that occurs between each of the pulses. In the illustratedembodiment the ratio of the duration of the first period to the durationof the second period (T(on)/T(off)) may be greater than 1. In otherwords, the duration of the first period (T(on)) may be greater than theduration of the second period (T(off)).

FIG. 7B is a schematic diagram of a regulator circuit 90 that isconfigured to provide voltages to the various lines of the column 70 ofthe memory device 30, in accordance with the technique discussedpreviously with regard to FIG. 7A. Specifically, the regulator circuit90 is configured to output a pulsed signal to a selected word line andto provide constant voltages to unselected word lines. The regulatorcircuit 90 includes a voltage input circuit 92 coupled in parallel tooutput 94, 96, 98, and 100 that are coupled to the word lines (WL0, WL1,WL2, and WL3). In the illustrated embodiment, the voltage input circuit92 includes a comparator 101, three resistors R1, R2, and R3, and twotransistors T1 and T2. A reference voltage (Vref) is input to one nodeof the comparator 93. Control signals program verify enable (pv_en) andread enable (read_en) are input to control gate of the transistors T1and T2, respectively.

Each of the outputs 94, 96, 98, and 100 includes an enable transistor102, a deselect transistor 104, and a reset transistor (RST) 106. Enablesignals (w10_en, w11_en, w12_en, and w13_en) are input to the controlgate of the enable transistors 102 on the outputs 94, 96, 98, and 100,respectively. Disable signals (w10desel_en, w11desel_en, w12desel_en,and w13desel_en) are input to the control gate of the deselecttransistor 104 on the outputs 94, 96, 98, and 100, respectively. RSTsignals (rst0, rst1, rst2, and rst3) are input to the reset transistor(RST) 106 on the outputs 94, 96, 98, and 100, respectively. Thewordlines can be grounded with the input to the reset transistor (RST)high.

FIG. 7C is a timing diagram that illustrates signals in accordance withan embodiment of method of operating the regulator circuit 90. Thediagram illustrates embodiments including, a read operation (e.g., WL1is pulsed between 0V and 1V) or a verify operation (e.g., WL1 is pulsedbetween 0V and 1.5V) where the output 96 is coupled to the selected cellvia the word line of the selected cell (WL1). The word line coupled tothe selected cell (WL1) is pulsed as discussed above with regard to FIG.7A. The enable signal (w11_en) is also pulsed between a low and a highstate, with a profile similar to that of the word line (WL1) coupled tothe selected cell. The RST signal (rst1) input to the reset transistor(RST) 106 of the outputs 96 is also pulsed during the period when theword line (WL1) and the enable signal (w11_en) are pulsed. However, theprofile of the RST signal (rst1) is the inverse of the word line (WL1)and the enable signal (w11_en). In other words, the RST signal (rst1) isin a low state while the word line (WL1) and the enable signal (w11_en)are in a high state, and the RST signal (rst1) is in a high state whilethe word line (WL1) and the enable signal (w11_en) are in a low state.The RST signals (rst0, rst2, and rst3), the enable signals of theunselected word line/outputs (w10_en, w12_en, and w13_en), and thedisable signal (w11desel_en) of the selected word line/outputs remain inthe low state. The disable signals (w10desel_en, w12desel_en, andw13desel_en) of the unselected word line/outputs are driven high (e.g.to about 6V) during the period when the world line (WL1) of the selectedcell is pulsed. In one embodiment, when the word line capacitance is 10picofarad (pF), a read time is 50 microseconds (us), the number of riseand falls (pulses) for the selected word line is 10, the average currentis 2 microamps (uA) ((10 pF×1 V)/(50 us×10)=2 microamps (uA)), less than1% of the total read current.

Once again, it is noted that the wordline (WL1) is pulsed during a firsttime period (T(on)) and maintained in a low state during a second period(T(off)) that occurs between each of the pulses. In the illustratedembodiment the ratio of the duration of the first period to the durationof the second period (T(on)/T(off)) may be greater than 1. In otherwords, the duration of the first period (T(on)) may be greater than theduration of the second period (T(off)).

FIG. 7D is a timing diagram that illustrates signals in accordance withanother embodiment of method of operating the regulator circuit 90. Thediagram illustrates embodiments including, a read operation (e.g., wordline (WL1) is pulsed between 0V and 1V) or a verify operation (e.g.,word line (WL1) is pulsed between 0V and 1.5V) and having an additionalvoltage spike (e.g., an excited pulse) that includes a period in whichthe cell voltage level is strong (e.g., above the read or program verifyvoltage). In such embodiments, a trap site can be filled with electrons,and the subsequent read or program verify can be performed with the trapsites filled. For example, an embodiment may include a read or verifypulse that includes consecutive pulses wherein one or more of the pulsesincludes a first period in which the cell is subject to a strong (e.g.,trap) voltage, a second period include the cell is subject to the reador verify voltage, and a third period in which the cell is turned off(e.g., subject to a low voltage, such as 0V). As is discussed in furtherdetail below, each of the periods may be varied in duration to achievedesired results.

In the illustrated embodiment, the signals are controlled in a similarmanner as to those discussed with regard to FIG. 7C. However, the wordline (WL1) includes a multi-level pulse having a spike in the voltagelevel substantially above level of the read voltage level (1V) and/orthe program verify voltage level (1.5V). For example, in the illustratedembodiment, the pulse of the word line (WL1) transitions to a trapvoltage level (5V) during a first time period (T(hard-on)), the voltagelevel of the pulse transitions to read voltage level (1V) and/or theprogram verify voltage level (1.5V) during a second time period(T(weak-on)), and the pulse ends as the voltage level transitions backto approximately 0V during a third time period (T(off)). The enablesignal (w11_en) is high during the second period (T(weak-on)), thedeselect line (w11desel_en) is high during the first period(T(hard-on)), the RST signal (rst1) is high during the third period(Toff), and the drain select line (SGD) transitions to a high stateduring the second period (T(weak-on)) and transitions back to a lowstate during the third period (T(off)). For example, in the illustratedembodiment, the drain select line (SGD) transitions from a low state toa high state after the transition of the word line (WL1) from the trapvoltage level (5V) to the read voltage level (1V) and/or the programverify voltage level (1.5V), and transitions back to a low state beforethe next pulse in the sequence. Discharge may occur when the word line(WL1) is 1V or 1.5V and the gate signal (SGD) is high.

It is also noted that in the illustrated embodiment the ratio of theduration of the second time period to the duration of the third timeperiod (T(weak-on)/T(off)) may be greater than 1. In other words, theduration of the second time period (T(weak-on)) may be greater than theduration of the third time period (T(off)). Further, the ratio of theduration of the first time period to the duration of the third timeperiod (T(hard-on)/T(off)) may be greater than 1 or less than 1. Inother words, the duration of the first time period (T(hard-on)) may begreater than or less than the duration of the third time period(T(off)).

FIG. 7E is a timing diagram that illustrates signals in accordance withanother embodiment of method of operating the regulator circuit 90. Thediagram illustrates an embodiment wherein prior to a read operation(e.g., word line (WL1) is pulsed between 0V and 1V) or a verifyoperation (e.g., word line (WL1) is pulsed between 0V and 1.5V) avoltage spike (e.g., an excited pulse) is applied to the selected wordline (WL1) to fill electrons in trap sites. In such embodiments, a trapsite can be filled with electrons, and the subsequent read or programverify can be performed with the trap sites filled. For example, in theillustrated embodiment, a first period (T(hard-on) includes the wordline (WL1) being excited to a trap voltage level (5V) during a firsttime period (T(hard-on)), the voltage level transitions back toapproximately 0V during a second time period, and the voltage level ofthe pulse transitions to read voltage level (1V) and/or the programverify voltage level (1.5V) during a third time period (T(weak-on)). Theenable signal (wl1_en) is high during the third period (T(weak-on)), thedeselect line (w11desel_en) is high during the first period(T(hard-on)), the RST signal (rst1) is high during the second period,and the drain select line (SGD) transitions to a high state during thethird period (T(weak-on)) and transitions back to a low state after thethird period (T(weak-on)). Discharge may occur when the word line (WL1)is 1V or 1.5V and the gate signal (SGD) is high.

FIG. 8 is a timing diagram that illustrates signals in accordance withanother embodiment of operating the memory device 30. The methodincludes providing a pulsed signal to the word line of the selected cell(WL1), as well as providing pulsed signals to other inputs of the column70. For example, the word line signals to the unselected cells (WL0,WL2, and WL3) and the select signals (SGS and SGD) are pulsed. Similarto the embodiments discussed with regard to FIG. 7A, during thedischarging period (Tbl), the word line to the selected cell (WL1) ispulsed between a low state, where the cell is off, and a high state,where the cell is on. For example, the high voltage level is about 1Vand the low voltage level is about 0V. While the word line signal to theselected cell (WL1) is supplied with the low voltage level, the memorycell connected to the word line (WL1) is accumulated to detrap electronsthat are trapped during the period where the word line signal to theselected cell (WL1) is at the high voltage level. Further, during thedischarging period (Tbl), the word line signals to the unselected cells(WL0, WL2, and WL3) and the select signals (SGS and SGD) are pulsedbetween a low state, where the cells/transistors are on, and a highstate, where the cells/transistors are off. In the illustratedembodiment, the high voltage level is about 5V and the low voltage levelis about 0V. Each of the signals (WL0, WL2, WL3, SGS and SGD) can begenerated via a regulator circuit that is the same or similar to theregulator circuit 90 discussed with to FIG. 7B.

FIG. 9A is a timing diagram that illustrates signals in accordance withanother embodiment of operating the memory device 30. The methodincludes a read operation that may detect whether the threshold voltage(Vt) of the selected cell is lower than a high voltage level or higherthan a low voltage level. In other words, the read operation determineswhether the threshold voltage (Vt) of the selected cell falls within agiven voltage range. The method includes providing a pulsed (e.g.,sequentially biased) signal (WL1) to the selected cell. The signal isdriven from a low state and is pulsed between a first high voltage leveland a second high voltage level. For example, during the dischargingperiod (Tbl), the pulsed word line signal to the selected cell (WL1)alternates between a first high voltage level of about 2V and a secondhigh voltage level of about 1V. While the word line signal to theselected cell (WL1) is supplied with the low voltage level, the memorycell connected to the word line (WL1) gets accumulated to detrapelectrons that are trapped during the period where the word line signalto the selected cell (WL1) is at the high voltage level. As mentionedpreviously, the method enables the memory device 30 to detect whetherthe threshold voltage (Vt) of the selected cell is below the first highvoltage level (2V) and/or higher than the second high voltage level(1V). The other signals operate similar to those discussed above withregard to FIGS. 6 and 7A. For example, the word line to the unselectedcells (WL0, WL2, and WL3) and the select gate signals (SGS and SGD) aredriven to a high voltage level, such as 5V, and are not pulsed (i.e.,they are held at a constant voltage level). Further, the dischargingperiod (Tbl) is extended such that the word line signal to the selectedcell (WL1) is in the high state for a total time that is approximatelyequal to the time period that the word line (WL1) is held high in anembodiment where the word line (WL1) is not being pulsed.

Further, it is noted that the wordline (WL1) is pulsed during a firsttime period (T(on)) and maintained in a low state during a second period(T(off)) that occurs between each of the pulses. In the illustratedembodiment the ratio of the duration of the first period to the durationof the second period (T(on)/T(off)) may be greater than 1. In otherwords, the duration of the first period (T(on)) may be greater than theduration of the second period (T(off)).

FIG. 9B is a schematic diagram of a regulator circuit 110 that mayprovide voltages to the various lines of the column 70, in accordancewith the techniques discussed with regard to FIG. 9A. Specifically, theregulator circuit 110 may output a pulsed signal to a selected word lineand output constant voltages to unselected word lines. The pulsed signalalternates between a first high voltage level and a second high voltagelevel (e.g., between 2V and 1V). The regulator circuit 110 includes avoltage input circuit 112 having a high-bias enable input (hbias_en) andan output coupled in parallel to outputs 114, 116, 118, and 120. In theillustrated embodiment, the voltage input circuit 112 includes acomparator 121, five resistors R4, R5, R6, R7, and R8, four AND logicgates AND1, AND2, AND3, and AND4, two inverters NOT1 and NOT2, and fourtransistors T3, T4, T5, and T6. A reference voltage (Vref) is input toone node of the comparator 93. Control signals program verify enable(pv_en), bias enable (hbias_en), and read enable (read_en) are coupledto the inputs of the AND logic gates AND1, AND2, AND3, and AND4 and theinverters NOT1 and NOT2, as depicted. Outputs of the four AND logicgates AND1, AND2, AND3, and AND4 are coupled to the control gate of thefour transistors T3, T4, T5, and T6, respectively. The outputs 114, 116,118, and 120 are coupled to the word lines WL0, WL1, WL2, and WL3,respectively. Each of the outputs 114, 116, 118, and 120 includes anenable transistor 122, a deselect transistor 124, and a reset transistor(RST) 126. Enable signals (w10_en, w11_en, w12_en, and w13_en) are inputto control gates of the enable transistors 122 on the outputs 114, 116,118, and 120, respectively. Disable signals (w10desel_en, w11desel_en,w12desel_en, and w13desel_en) are input to control the gate of thedeselect transistor 124 of the outputs 114, 116, 118, and 120,respectively. RST signals (rst0, rst1, rst2, and rst3) are input to thecontrol gate of the reset transistor (RST) 126 of the outputs 114, 116,118, and 120, respectively.

FIG. 9C is a timing diagram that illustrates signals in accordance withan embodiment of operating the regulator circuit 110 of FIG. 9B. Thediagram illustrates an embodiments of a read operation (e.g., anoperation where WL1 is pulsed between 2V and 1V) and/or verify operation(e.g., an operation where WL1 is pulsed between 2V and 1.5V) where theoutput 116 is coupled to the selected cell via the word line WL1. Theword line of the selected cell (WL1) is pulsed as discussed above withregard to FIG. 9A. The enable signal (w11_en) is driven to a highstate/voltage level and is held constant while the word line of theselected cell (WL1) is pulsed. The high-bias enable input (hbias_en) ispulsed between a high voltage level and a low voltage level, with aprofile similar to that of the word line of the selected cell (WL1). TheRST signals (rst0, rst1, rst2, and rst3), the enable signals of theunselected word line/outputs (w10_en, w12_en, and w13_en), and thedisable signal (w11_desel_en) of the selected word line/outputs remainat a low voltage/state during the period when the word line of theselected cell (WL1) is pulsed. The disable signals (w10desel_en,w12desel_en, and w13desel_en) of the unselected word line/outputs aredriven high during the period when the world line of the selected cell(WL1) is pulsed. In one embodiment, when the word line capacitance is 10pF, a read time is 50 us, the number of rise and falls (pulses) for theselected word line is 10, the average current is 2 uA ((10 pF×1 V)/(50us×10)=2 uA, less than 1% of the total read current.

Once again, it is noted that the wordline (WL1) is pulsed during a firsttime period (T(on)) and maintained in a low state during a second period(T(off)) that occurs between each of the pulses. In the illustratedembodiment the ratio of the duration of the first period to the durationof the second period (T(on)/T(off)) may be greater than 1. In otherwords, the duration of the first period (T(on)) may be greater than theduration of the second period (T(off)).

FIG. 9D is a timing diagram that illustrates signals in accordance withanother embodiment of method of operating the regulator circuit 90. Thediagram illustrates an embodiment including, a read operation (e.g.,word line (WL1) is pulsed between 1V and 2V) and/or a verify operation(e.g., word line (WL1) is pulsed between 1.5V and 2.5V) and having anadditional voltage spike (e.g., excited pulse) that occurs at thebeginning of the high state of the pulse. In such embodiments, a trapsite can be filled with electrons during the spike, and the subsequentread or program verify can be performed with the trap sites filled.

In the illustrated embodiment, the signals are controlled in a similarmanner as to those discussed with regard to FIG. 9C. However, the wordline (WL1) includes a multi-level pulse that first transitions to a trapvoltage level, and returns to the high voltage level before returning tothe low voltage level (e.g., above 0V) between the pulses. For example,in the illustrated embodiment, the pulses of the word line (WL1)transition from 0V or the low voltage level (1V or 1.5V) to the trapvoltage level (5V) during a first time period (T(hard-on) (during whichthe trap sites may be filled with electrons), the voltage level of thepulse transitions back to the high level (2V or 2.5V) of the read orverify voltage pulse during a second period (T(weak-on)), and voltagelevel transitions back to the low voltage level (1V or 1.5V) of the reador verify voltage pulse (e.g., the voltage level between the pulses)during a third period (T(off)).

The enable signal (w11_en) is low during the first period (T(hard-on),transitions to high during the second period (T(weak-on) and transitionsback low during the third period (T(off)). The deselect line(w11desel_en) is high during the first period (T(hard-on)) and is lowduring the second period (T(weak-on)) and the third period (T(off)). TheRST signal (rst1) transitions to high after the series of pulses (e.g.,when the word line (WL1) returns to 0V). The high-bias enable input(hbias_en) is high during the second period (T(weak-on). The drainselect line (SGD) transitions to a high state during the second period(T(weak-on)) and transitions back to a low state during the third period(T(off)). The drain select line (SGD) transitions to a high state afterthe deselect line (w11desel_en) goes low, and the drain select line(SGD) transitions to a low state before the deselect line (w11desel_en)goes high. Discharge may occur when the word line (WL1) is 2V or 2.5Vand the gate signal (SGD) is high.

It is also noted that in the illustrated embodiment the ratio of theduration of the second time period to the duration of the third timeperiod (T(weak-on)/T(off)) may be greater than 1. In other words, theduration of the second time period (T(weak-on)) may be greater than theduration of the third time period (T(off)). Further, the ratio of theduration of the first time period to the duration of the third timeperiod (T(hard-on)/T(off)) may be greater than 1 or less than 1. Inother words, the duration of the first time period (T(hard-on)) may begreater than or less than the duration of the third time period(T(off)).

FIG. 10 is a timing diagram that illustrates another embodiment of amethod of operating the memory device 30, in accordance with the presenttechniques. The method includes providing a constant voltage to theselected cell, and pulsing other signals coupled to the column 70.Specifically, the embodiment includes pulsing the source-line (SL) andthe drain-select line (SGD), while the voltage level of the signals tothe cells 72 (WL0, WL1, WL2, and WL3) and the signal to the sourceselect gate transistor 78 (SGS) remain at a constant voltage level.During the discharging period (Tbl) the source line (SL) is pulsedbetween a low state and a high state and the drain-select line (SGD) ispulsed between a high state when the source line (SL) is low and a lowstate when the source line (SL) is high. For example, in the illustratedembodiment, the source line (SL) alternates between a low voltage levelof about 0V and a high voltage level of about 2V, and the source-line(SL) alternates between a low voltage level of 0V and a high voltagelevel of 5V. The source-line (SL) and the drain select line (SGD) arepulsed between high and low states during the period when the word lines(WL0, WL1, WL2, and WL3) and the source select line (SGS) are driven toa high state. For example, the word line of the selected cell (WL1) isdriven to 1.0V during a read operation or 1.5V during a verifyoperation, and the remaining word lines to the unselected cells (WL0,WL2, and WL3) and to the source select line (SGS) are driven to 5V.During the period when the source line (SL) is driven high (e.g., to2V), the cells connected to the word line of the selected cell (WL1) areaccumulated to detrap electrons that are trapped in cell. It is againnoted, that the discharging period (Tbl) is extended for a period suchthat the word line signal of the selected cell (WL1) is in the highstate for a total time that is approximately equal to the time periodthat the word line (WL1) is held high in an embodiment where the wordline (WL1) is not being pulsed (e.g., the time period Tbl of FIG. 6). Inone embodiment, when the source line (SL) and the drain select line(SGD) capacitance are 1 nanofarad (nF) and 10 pF, respectively, a readtime is 50 us, the number of rise and falls (pulses) for the selectedword line is 10, the current efficiency of a 5V generation charge pumpcircuit is 20%, the average current is 500 uA ((1 nF×2V+(10pF×5V)/2)/(50 us×10)=2 uA, approximately 10% of the total read current.

Although the previous embodiments are discussed in the context of a NANDflash memory device 30, the systems and methods can be applied to othertypes of memory where stored data is read by cell current as well. FIGS.11A-11E include schematic diagrams of other memory devices that mayemploy techniques similar to those discussed above. For example, FIG.11A illustrates a NOR flash memory cell 130. The NOR flash memory cell130 includes a transistor 131 coupled between a bit line (BL) and asource line (SL) of a memory array, and a control gate of the transistor131 coupled to a word line (WL) of the memory array. FIG. 11Billustrates a static random access memory (SRAM) cell 132. The SRAM cell132 includes two cross-coupled inverters 134 and 136 and two accesstransistors 138 and 140. A control gate of each of the transistors 138and 140 is coupled to a word line (WL) of a memory array. Further, thesource and drain nodes of the access transistors 138 and 140 are coupledbetween the cross-coupled inverters 134 and 136 and complementary bitlines BL and /BL of the memory array. FIG. 11C illustrates a floatingbody cell 142. The floating body cell 142 includes a transistor 144coupled between a bit line (BL) and a source line (SL) of a memoryarray, and a control gate of the transistor 144 coupled to the word line(WL) of the memory array. FIG. 11D illustrates a phase change memorycell 146. The phase change memory cell 146 includes a transistor 148coupled between a bit line (BL) and a source line (SL) of a memoryarray, and a phase change material 150 disposed between a drain node ofthe transistor 148 and the bit line (BL) of the memory array. Further, acontrol gate of the transistor 148 is coupled to a word line (WL) of thememory array. FIG. 11E illustrates a magnetoresistive random accessmemory (MRAM) cell 152. The MRAM cell 152 includes a transistor 154coupled between a bit line (BL) and a source line (SL) of a memoryarray, and a magnetic material 156 disposed between a drain node of thetransistor 154 and the bit line (BL) of the memory array. Further, acontrol gate of the transistor 154 is coupled to a word line (WL) of thememory array.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the following appended claims.

1. A system, comprising: a memory device, comprising: a plurality ofmemory cells electrically coupled together via a plurality of accesslines and a plurality of data lines; and a regulator circuit coupled tothe plurality of access lines, and configured to sequentially bias atleast a selected one of the plurality of the access lines between afirst voltage level and a second voltage level at least three timesduring a cycle of a read operation or a verify operation.
 2. The systemof claim 1, wherein the at least one of the access lines comprises anaccess line coupled to the control gate of a selected one of theplurality of memory cells.
 3. The system of claim 1, wherein theregulator circuit is configured to provide a constant third voltagelevel to unselected access lines of the plurality of the access linesduring a period when the selected one of the plurality of the accesslines is sequentially biased.
 4. The system of claim 1, wherein thememory device comprises a flash memory device.
 5. The system of claim 1,wherein the memory device comprises a NAND flash memory device.
 6. Thesystem of claim 1, wherein the regulator circuit comprises a voltageinput circuit configured to provide the first voltage level and thesecond voltage level to the plurality of access lines.
 7. A system,comprising: a memory device, comprising: a memory array comprising aplurality of memory cells coupled together via a plurality of word linesand a plurality of bit lines; and a regulator circuit configured tooutput pulsed voltages to a selected one of the plurality of word linesand to provide constant voltages to unselected ones of the plurality ofword lines during a read operation or a verify operation.
 8. The systemof claim 7, wherein the regulator circuit comprises a voltage inputcircuit configured to provide the pulsed voltages and the constantvoltages.
 9. The system of claim 8, wherein the regulator circuitcomprises a comparator configured to receive an external referencevoltage at a first input and configured to receive a feedback signal ata second input.
 10. The system of claim 9, wherein an output of thecomparator is coupled to the plurality of word lines.
 11. The system ofclaim 7, wherein the regulator circuit is configured to output pulsedvoltages more than once during a cycle of a read operation or a verifyoperation.
 12. The system of claim 7, wherein the regulator circuit isconfigured to output pulsed voltages alternating between a first voltagelevel and a second voltage level.
 13. The system of claim 12, whereinthe second voltage level is approximately 0 volts.
 14. The system ofclaim 12, wherein the first voltage level is greater than the secondvoltage level, and wherein the second voltage level is greater thanapproximately 0 volts.
 15. The system of claim 12, wherein the pulsedvoltage is at the first voltage level at least as long as the pulsedvoltage is at the second voltage level.
 16. The system of claim 7,wherein the regulator circuit is configured to output pulsed voltagesalternating between a first voltage level, a second voltage level and athird voltage level.
 17. The system of claim 16, wherein the thirdvoltage level is approximately 0 volts.
 18. The system of claim 16,wherein the first voltage level is greater than the second voltagelevel, and wherein the second voltage level is greater than the thirdvoltage level.
 19. The system of claim 18, wherein the third voltagelevel is greater than approximately 0 volts.
 20. The system of claim 16,wherein the second voltage level comprises a read voltage or a verifyvoltage.
 21. The system of claim 20, wherein the first voltage level isgreater than the read voltage or the verify voltage.
 22. A system,comprising: a memory device, comprising: a plurality of floating gatetransistors, each comprising a respective control gate; a senseamplifier coupled to the plurality of floating gate transistors andconfigured to sense a response during an operation cycle of the memorydevice to determine data stored on at least one of the plurality offloating gate transistors; and a regulator circuit configured to outputa series of pulses during the operation cycle to the control gate of atleast one of a plurality of floating gate transistors, wherein theseries of pulses comprises a plurality of alternating low state levelsand high state levels.
 23. The system of claim 22, wherein the regulatorcircuit is configured to provide a series of pulses during the oneoperation cycle to a respective control gate of at least two of theplurality of floating gate transistors.
 24. The system of claim 22,wherein the high state levels are greater than 0 volts and less than 2.0volts.
 25. The system of claim 22, wherein the series of pulsescomprises a plurality of alternating low state levels, medium statelevels and high state levels.
 26. The system of claim 22, wherein thelow state levels are approximately 0 volts.